Subaudio electrical filter



Feb. 23, 1954 R. w. GILBERT 2,670,460

SUBAUDIO ELECTRICAL FILTE R Filed Jan. 9, 1951 3nventor:

Patented Feb. 23, 1954 'SUBAUDIO ELECTRICAL FILTER Boswell W. Gilbert, Montclair, N. J., assignor to Weston Electrical Instrument Corporation, Newark- N. J a corporation of New Jersey Applicationllanuary 9, 1951, Serial No. 205,126

This invention relates to'sub-audio filters for inclusion in electrical networks and .more palticularly to filters including .elements havin the general characteristics of direct current measuring instruments and which have an effective transmission in sub-audio ranges far below those which can be attained with electrical impedances of commercially practical values. It is common practice to transmit a plurality of communications'or signals over a single electrical channel, for example to transmit alternatingcurrent signals of different audio. frequencies over a conventional 60 cycles per second power distribution system selectively to energizeswitch equipment at remotely located sub-stations. The several signals of different audio frequencies are diverted from the distribution system and into their individual control circuits by means of electrical wave filters or sensitive relays .resonant at an audio frequency. Such tuned relays are not sensitive at low and sub-audio fre quencies, and it is neither practical'nor possible to construct an electrical wave filter for use at sub-audio frequencies of the order of, for example, one cycle per second. At low frequencies, for example one cycle per-second, inductors of practical dimension have too low a Q factor (ratio of reactanceto resistance) for reasonably narrow band widths because of the relatively high required inductance. For instance, a 50 henry inductor having-a Q of 50 at one cycle per second would weigh hundreds of pounds.

It; is known that. in general, all electrical, mechanical and thermal systems, andcombinations thereof, have exact analogs in terms of each other because they follow the same fundamentallaws. The mathematical analysis of an electrical network which includes an electromechanical element may be conveniently carried out by expressing the element in terms of an equivalent electrical sub-network, and this technique has been used quite extensively in physics. It is common practice, for example, to express devices such as sound transducers, vibrators and other electromechanical elements in terms of their equivalent networks. Objects of the present-inventionare to provide sub-audio electrical filters in which the equivalents of inductors and capacitors of impractically high values are furnished by electromechanical devices. Objects are to provide subaudio electrical filters in which direct current instruments afiord. high selectivity .at frequencies. of-and below about cycles per second. More specifically, an object is to provide a sub-audio 3 Claims. (Cl. '333-.'71)

filter including a pair of mechanical oscillatory systems resonant at the same frequency and coupled through a spring of low torque, whereby the filter has the characteristics of a flat-topped band pass filter with sharp cutoff. A further specific object'is to provide a relay resonant at a sub-audio frequency'and comprising an oscillating mass carrying a contact arm'and coupled by a spring to the moving system of a direct current instrument.

These and other objects and'theadvantages of the invention will be apparent from the following specification when "taken with the accompanying drawings, in which:

Figs. 1a and 1b are idealized or schematic elevation and plan views, respectively, of a permanent magnet-moving coil instrument such as employed in embodiments of the invention;

Fig. 2 is a circuit diagram of an equivalent electrical network;

Fig. 3 is a graph showing the frequency selectivity of the network;

Fig. 4 is a schematic perspective view of a band pass filter according to the invention;

Fig. 5 is a diagram of the equivalent electrical network; 7

Fig. 6 is a graphshowing the selectivity of the network; and

Fig. 7 is a schematic perspective view of a diflerent type "of secondrstage for the bandpass filter.

In Figs. 1a and 1b of the drawings, the reierence numeral l identifies the moving coil of a permanent magnet-moving coil electrical in.

strument such as commonly employed for the measurement of direct current and direct .cur-

rent voltages, the coil being supported in jewel bearings 2 for angular-displacement in the gaps between the polar faces of a permanent magnet Sand the adjacent surfaces of a soft iron core 4. The inner ends of spiral springs 5, '5 which return the coil to a preselected position in the absence of a ,coil current are anchored to the staffs 6, .6 and constitute circuit connections between the coil i and the instrument terminals I, 1.

r The operation of the .instrument as an element of a filter for sub-audio frequencies can be best understood by considering the frequency selectivity of the equivalent electrical network, Fig. 2, in which the inductance L represents the compliance U of the springs 5, .5, the capacitance C represents the moment of inertia M of the moving system, the shunt resistance R: repre-- sents the'losses both frictional and electrical oi the moving system, and the series resistance Re represents the direct current resistance measured at the instrument terminals.

The mechanical parameters of moment of inertia and spring compliance are related to their electrical equivalents by the flux linkage constant F of the mechanism, as

[LC M U 1 from which L= UF (3) and wherein the constant F is the product of the movable coil area. (A), the number of coil turns (N) and the magnetic flux density (B) FEBAN (5) From (8) it is apparent that for values of Q in the order of 10 or higher Wn and We are virtually equal, which is usually the case.

The selectivity of the parallel resonant circuit is a function of Q as:

ton-

wherein i is the circulating current at frequency W, relative to the current in which would circulate at frequency We. The selectivity is qualitatively that of a single resonant circuit, as shown by curve S of Fig. 3.

The selectivity curve may be given a fiat top. which is appropriate for a bandpass filter, by coupling the instrument to a second mechanical system which is resonant at or near the frequency of natural resonance of the instrument. As shown in Fig. 4, the second mechanically resonant system is also a direct current-moving coil instrument, and, for convenience of description and analysis, the instruments are assumed to be of identical construction. Elements of the instrument constituting the first stage of the filter, i. e. the instrument at the left in Fig. 4, are idenified by the reference numerals of Fig. 1, and the corresponding elements of the instrument constituting the second stage are identified by the corresponding but primed numerals. The terminals 1, 'l of the coil I of the first instrument are connected to a transmission line L' of a power or communication system upon which supervisory signals of the sub-audio frequency may be imposed for the selective operation of various devices, not shown, which may be connected across the terminals 1', 1' of the coil l of the second instrument.

The moving systems are coupled mechanically by a spiral spring 8 having a compliance Um which is represented in the equivalent electrical network, Fig. 5, by the mutual inductance Lm. The electrical equivalent of each instrument is identical with the network of Fig. 2 since it is assumed that both instruments are identical with the Fig. 1 instrument. The elements of the equivalent electrical network of Fig. 5 are therefore identified by the corresponding characters of Fig. 2, but will not be described in detail.

The degree of coupling between the stages may be expressed as a coupling factor K:

KEL/Lm (10) The selectivity characteristic of such a coupled circuit may be expressed as:

(12) wherein KQ is the coupling coefficient."

The condition where the single peak divides into two peaks appears when K=1/Q, or KQ=1 (13) which condition is termed critical coupling.

Comparing the selectivity Equation 9 for a single circuit with Equation 11 for coupled circuits, it is apparent that two coupled circuits would have a considerably higher selectivity than either circuit used separately. Also, when the coupling coefficient KQ is somewhat larger than unity so that the double peaked condition is present but not excessive, the selectivity characteristic S approximates a fiat-topped pass band which is ideal for most purposes, see Fig. 6. Such a coupled system is known as a bandpass filter" because of its relatively flat response over a range of frequency, with a sharp attention outside of the pass band range.

The factor (W1Wz) /Wo of Equation 12 is usually termed the "relative peak bandwidth. Or alternatively the practical working bandwidth may be considered as the width between the points on the selectivity curve that equal in level the point in the center of the curve at W0. These points are separated farther apart than W1 and W2 by \/2. These practical working bandwidths are usually the factors of interest and are termed the relative bandwidth and the bandwidth respectively.

ami -46o l bandwidth 5 ,mm- W2) =WQK[2( 1 *Q)] relative bandwidth 5 1 These equations involving .the equivalent electrical parameters 6, I and It], may be converted to the mechanical parameters by substitution of 3, 4 and 5.

Equation 6 becomes:

Q The losses are in practice still mostly electrical so R1 remains an electrical parameter. Likewise Equation 7 becomes:

W,E MU 2 (17 and Equation 10 becomes:

KEU/Um (18) wherein Um is the compliance of the coupling spring.

The coefficients Q and K and the frequency factors are equally applicable to mechanics and electricity, and serve mainly to simplify expressions that would become unduly cumbersome by substitution. Equations 8, 9, and 11 to 15 are therefore considered as appropriate to mechanics by use of Q, K and We as defined by Equations 16 through 18.

The transmission-frequency characteristic S as derived from the analysis of the equivalent electrical network is, by analogy, the oscillation amplitude-frequency characteristic of the coupled electrical instruments of Fig. 4. The coils I, I of the filter stages each have an undamped resonant frequency of oscillation We which, :by construction in accordance with the foregoing equations, is to be in the range of sub-audio frequencies, say 10 cycles per second or less if the filter is not housed in an evacuated envelope. The limits of the useful range of sub-audio frequencies have not been fully investigated but, from present knowledge of moving coil structures and frictional effects in permanent magnet-moving coil instruments as currently manufactured commercially, it would appear that the usable range extends from about 10 cycles per second down to about 0.1 cycle per second. At higher frequencies, air friction becomes serious, and at lower frequencies pivot friction becomes serious. This frequency range provides a relatively large number of signal channels as the instruments have relatively low frictional losse and thus a relatively high mechanical Q value which, in turn, provides a very narrow band width and high selectivity.

A plurality of sub-audio filters may therefore be provided upon a transmission line L to control various devices in accordance with supervisory signals of different sub-audio frequencies which are imposed upon the line by appropriate signal generators. The coil 1 of the first stage of each filter will respond, or be set into oscillation, onlyby signals of approximately its particular undamped resonant frequency W0, and the oscillation of coil i will act through the coupling spring 8 to set the coil I of the second filter stage into oscillation. The function of the second stage is,

of'course, to -provide a second filtering which clamps out the effects of low amplitude. oscillations of the coil I by signals of frequenciesad'jacent, but outside of, the selected bandpass range. The oscillation of the coil I within the magnetic field established by the permanent magnet 3' generates'an alternating current which is delivered to a tuned relay, not shown, connected across the output terminals I, 1', thecurrent being of a frequency within the bandpass range.

'It is not essential that the second stage of the filter be a current generator for delivering a. doubly-filtered electrical signal to a tuned relay. As shown in Fig. '7, the second filter stage may comprise a staff ll supported in jewel bearings l2 in axial alinement with the bearings 2 of the coil l of the first filter stage, the staff having an arm l3 to which the outer end of a spiral spring 8 on the staff of coil I is connected. The staff I l carries a relay contact arm [4 for cooperating with stationary relay contacts l5, and a disc l6 of such mass that the spiral spring H which affords a current connection to the contact arm l4 tunes the moving system to the resonant frequency W0 of the input instrument stage of the filter. The relay circuit includes a current source [8, indicated schematically as a battery, and a load device I9 connected between the stationary relay contacts l5 and the spring H. The manner in which the relay responds only to sharply filtered signals of the sub-audio frequency We will be apparent from the above explanation of the operation of the mechanically equivalent bandpass filter of Fig. 4.

It is to be understood that the invention is not limited to the apparatus a structurally and schematically illustrated, and as described. It is of course possible to employ magnetic core instruments in place of the external magnet instrument illustrated in Fig. 1, and the stages of the filter may be in mechanical parallel arrangement in place of the illustrated axial alinement. Additional stages may be provided for higher selectivity but, in general, two stages afford satisfactory selectivity, for practical purposes. These and other changes which may occur to those familiar with the design and construction of electrical filters fall within the spirit and scope of the invention as set forth in the following claims.

I claim:

1. In an electrical filter, the combination with a permanent magnet-moving coil intrument comprising a permanent magnet for establishing a magnetic field, a moving system including a coil supported for oscillation in said magnetic field in response to currents of sub-audio frequency therethrough, and spiral springs constituting current connections to said coil and tuning said moving system to mechanical resonance at a preselected sub-audio frequency; of a second moving system supported for oscillatory motion independently of said first moving system, spiral spring means tuning said second moving system to mechanical resonance at substantially said preselected frequency, and resilient means coupling said second moving system to said first moving system for actuation thereby, said second moving system comprising a coil movable in a unidirectional magnetic field, said spiral spring means constituting current connection to terminals across which an alternating voltage of substantially said preselected frequency is developed upon oscillation of said second moving system.

2. An electrical relay or repeater selectively responsive to an input voltage of a preselected frequency within a range of sub-audio frequencies to develop an output current of the preselected frequency, said relay or repeater comprising a, pair of permanent magnet-moving coil instruments, each instrument including a moving system comprising a coil and spiral springs constituting current connection to the coil and tuning the moving systems each independently to mechanical resonance at the preselected frequency, pairs of input and output terminals to which said spiral springs of the respective instruments are connected, and a spiral spring connecting said moving systems.

3. The invention as recited in claim 2, wherein the compliance of said coupling spiral spring has a value which provides more than critical coupling between the moving systems.

ROSWELL W. GILBERT.

References Cited in the file of this patent UNITED STATES PATENTS Number 5 786,696 1,673,254 1,719,484 2,1 12,560 2,184,321 10 2,560,257 2,622,168

Number Name Date Vreeland Apr. 4, 1905 Leeson June 12, 1928 Norton July 2,1929 Davies Mar. 29, 1938 Soller Dec. 26, 1939 Sias July 10, 1951 Shields et a1. Dec. 16, 1952 FOREIGN PATENTS Country Date Great Britain Sept. 5, 1946 OTHER REFERENCES Terman Radio Engineering (Third edition), 1947, pp. 58-65, 69 and 70. 

